i8o RELATIONS OF SOIL TO HEAT 



Greenwich, Brussels. Edinburgh. 



3-2 feet 21-4 ... 19-3 ... 16-8 



6-4 15.l 10.2 



12-8 9.3 ... 8.l ... 5.0 



25.6 3-4 ... 2.0 ... 1.3 



In none of these instances is the point reached at which no 

 variation of temperature is observed. H. Fritsche (Reper- 

 torium fur Meteorologie, 1872) has calculated from the ob- 

 servations made at Upsala, Edinburgh, Paris, Strassburg, and 

 Zurich, the average depths at which the annual variation will 

 amount to i-o, o-i, and ooi C. ; his results are as follows : 



Annual Temperature Variation. Average Depth of Soil. 



Centigrade. Fahrenheit. Metres. Feet. 



1.00 ... 1-80 ... 8.55 ... 28.04 



0-10 ... 0.18 ... 15.35 ... 50-34 



0.01 ... 0.02 ... 22-37 ... 73-36 



The variations of temperature immediately below the sur- 

 face depend very much on the exposure at the surface. At 

 observatories a hut is usually placed over the soil ther- 

 mometers for the protection of the stems which rise above 

 the surface. The soil in which the shallow thermometers are 

 placed is thus neither heated by direct sunshine, nor exposed 

 to night radiation. The extent of variation at a short distance 

 below the surface is thus much smaller than would occur in 

 an open field. Schubler's determinations of midday tem- 

 peratures at Geneva (Table XXVI) show a variation between 

 the monthly means of 43-6 at 3 inches, and 3O5 at 4 feet. 

 The mean monthly temperatures in Pennsylvania (Table XXX) 

 also show a variation of 4o-8 at a depth of 6 inches. 



Soil being a bad conductor of heat the alterations of tem- 

 perature at the surface pass but slowly downwards, and affect 

 the temperature of the subsoil long after the effect on the 



